Interpretive Summary: The synergism of diammonium phosphate (DAP) and urea in imparting flame resistance to cotton-based materials is well known, but most studies have been focused on bleached/scoured cotton woven fabrics. Since flame resistance is strongly dependent on the surface characteristics and construction of fabric, it is necessary to characterize the synergistic function of DAP and urea for greige cotton nonwoven fabric on a quantitative basis and study the thermal degradation of flame retardant FR greige cotton. This study demonstrated that greige cotton nonwoven fabric is an effective substrate in producing fire barriers at a fixation of 0.8% phosphorus of DAP and with additional nitrogen from urea. The results of thermogravimetry, ATR-FTIR, XRD, 1H --- 13C CP/MAS NMR, and SEM indicated that urea not only facilitated the phosphorylation of cellulose, but also introduced carbamate groups in cellulose upon heating. The addition of urea also increased the decomposition rate of glycosyl units at a slightly higher temperature and formed foam on the charred fiber to protect the substrate from heat. The information derived from this study is expected to contribute to the utilization of greige cotton nonwoven fabric in the application of fire barrier products and have the benefits of low-cost production and environmental protection.

Technical Abstract:
This study showed that greige cotton nonwoven fabric can effectively be flame retardant by applying the phosphorus of diammonium phosphate (DAP) in concentrations as low as 0.8% with the addition of urea. At such a low concentration of phosphorus, the char length and limiting oxygen index (LOI) value were continuously decreased and increased, respectively, as the concentration of urea increased. The effect of urea additive on the thermal decomposition process of flame retardant greige cotton nonwoven fabric was investigated by thermogravimetry, ATR-FTIR, XRD, 'H-713C CPIMAS NMR, and SEM. The results indicated that, upon heating, urea not only facilitated the phosphorylation reaction of DAP with cellulose but also introduced carbamate groups into cellulose to decrease the degree of crystallinity of the cellulose prior to the decomposition of the crystalline region. Compared with DAP treatment alone, the addition of urea accelerated the decomposition of glycosyl units at a slightly higher temperature, which resulted in a slight increase of weight loss and decrease of char yield. The char morphology observed after LOI tests indicates that urea released nonflammable gases, which blew the carboneous char into the foam protecting the substrate.